Click here to enlarge imageThe first step in destabilization of the anaerobic process begins with an increase of the VFA concentration in the bio-digester. Due to a high organic loading, failure of the heating or inhibition of the methanogenic – or methane-producing – bacteria (lack of nutrients or the presence of toxic compounds), VFA are more rapidly formed than degraded. While the concentration of VFA is increasing, the bicarbonate concentration will decrease and the CO2 concentration in the biogas will increase. If the biogas is used for energy recovery, an unstable gas production will result in a less efficient energy production.
As long as bicarbonate in the water buffers and protects the Bio-digester, organic molecules will be degraded. If no action is taken, however, the VFA concentration will continue to increase and, when all the bicarbonate is consumed, the pH of the bio-digester will drop very fast. Such a drop in the pH will cause a destruction of the bacterial ecosystem and, finally, the methanogenic bacteria are killed. Recovery of the bio-digester can take several months.
Thus, it's clear that a stable process is very important. To avoid instability of the anaerobic process, a bio-digester should be controlled via measurements of VFA, which is one of the most sensitive parameters, in combination with measurements of bicarbonate, alkalinity and pH.
In industry, though, few laboratories are equipped for VFA measurements because the standard techniques like gas chromatography (GC) require specialized equipment, high investment, significant operating costs and highly trained personnel. Because of the complex sample preparation, GC analysis is time consuming. If the process needs to be monitored very frequently (every 15 minutes), in case the bio-digester is being restarted after a period of non-activity, it's almost impossible to do so with GC.